Biochemistry of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 34

NAD to NADH. This step is catalyzed by isocitrate dehydrogenase. α -Ketoglutarate is

converted to succinyl CoA by -ketoglutarate dehydrogenase, along with the removal of

another molecule of carbon dioxide and the conversion of NAD to NADH. Succinate, the
next product, is formed from succinyl CoA by the action of succinyl CoA synthetase that
involves the removal of the CoA moiety and the conversion of ADP to ATP. Through
these steps, the complete oxidation of the acetyl CoA moiety has been achieved with the
removal of two molecules of carbon dioxide. Thus, succinate is a four-carbon organic
acid. Succinate is further converted to fumarate and malate in the presence of succinate
dehydrogenase and fumarase, respectively. Malate is oxidized to oxaloacetate by the enzyme
malate dehydrogenase along with the conversion of NAD to NADH. Oxaloacetate then can
combine with another molecule of acetyl CoA to repeat the cycle. The reducing power
generated in the form of NADH and FADH (succinate dehydrogenation step) is used for
the biosynthesis of ATP through the transport of electrons through the electron transport
chain in the mitochondria.

α

3.3.1.5 Gluconeogenesis
Several fruits store large amounts of organic acids in their vacuole, and these acids are
converted back to sugars during ripening, a process termed as gluconeogenesis. Several
irreversible steps in the glycolysis and citric acid cycle are bypassed during gluconeogenesis.
Malate and citrate are the major organic acids present in fruits. In fruits such as grapes,
where there is a transition from a sour to a sweet stage during ripening, organic acids
content declines. Grape contains predominantly tartaric acid along with malate, citrate,
succinate, fumarate, and several organic acid intermediates of metabolism. The content of
organic acids in berries can affect their suitability for processing. High acid content coupled
with low sugar content can result in poor quality wines. External warm growth conditions
enhance the metabolism of malic acid in grapes during ripening and could result in a high
tartarate/malate ratio, which is considered ideal for vinification.
The metabolism of malate during ripening is mediated by the malic enzyme, NADP-
dependent malate dehydrogenase. Along with a decline in malate content, there is a concomi-
tant increase in the sugars suggesting a possible metabolic precursor product relationship
between these two events. Indeed, when grape berries were fed with radiolabeled malate, the
radiolabel could be recovered in glucose. The metabolism of malate involves its conversion
to oxaloacetate mediated by malate dehydrogenase, the decarboxylation of oxaloacetate to
phosphoenol pyruvate catalyzed by PEP-carboxykinase, and a reversal of glycolytic path-
way leading to sugar formation (Ruffner et al., 1983). The gluconeogenic pathway from
malate may contribute only a small percentage (5%) of the sugars, and a decrease in malate
content could primarily result from reduced synthesis and increased catabolism through
the citric acid cycle. The inhibition of malate synthesis by the inhibition of the glycolytic
pathway could result in increased sugar accumulation. Metabolism of malate in apple fruits
is catalyzed by NADP-malic enzyme that converts malate to pyruvate. In apples, malate
appears to be primarily oxidized through the citric acid cycle. Organic acids are important
components of citrus fruits. Citric acid is the major form of the acid followed by malic acid
and several less abundant acids such as acetate, pyruvate, oxalate, glutarate, and fumarate. In
oranges, the acidity increases during maturation of the fruit and declines during the ripening
phase. Lemon fruits, by contrast, increase their acid content through the accumulation of
citrate. The citrate levels in various citrus fruits range from 75 to 88%, and malate levels